Question

How many terms of an ap should be taken for their sum to be 120.If its 3rd term is 9and the diff betwen 7th and 2nd is 20

Answer 1

given 3rd term = a+2d=9----(1)

and a+6d - (a+d)= 20

=> 5d= 20

=>d=4

sub d=4 in 1 you get a =1

now use the formula n/2{2a+(n-1)d}= 120 sub values of a and d and find n